This question pertains just as well I presume to all types of amps be they SS, tube or hybrids but I am posting it here in Tubes because this is what interests me more. I've never given this issue much thought up until now but I would like to know from the seasoned veterans in what ways if at all possible, we can design for low IM distortion.

I understand that when designing an amp we can exert some sort of control on the THD products of the circuit by the choice of the load line, bias point, number of stages weather the amp is SE or PP etc. I am talking open loop here and I'm excluding the use of NFB since (at least where THD is concerned) application of feedback alone has a great effect in reducing distortion.

What I would like to know is if there are similar ways that can be used to effect a desired IM distortion profile to a particular design. In other words, what are the mechanisms that govern how much IM distortion is there in a particular circuit? If someone wanted to design an amp where the overriding concern above all else was low IM distortion how would a good designer go about his business? I hope this is not too much of a loaded question.

Last edited by musical noise; 27th April 2011 at 10:03 PM.
Reason: spelling

it is possible to construct a circuit with low THD but generates high IMD - but the methods are not much used in amps intended to be linear

Prof Cherry gave his 3 amplifier distortion reduction options:

reduce signal to bias ratio - use less of the nonlinear curve by wasting lots of power in heavy bias

cancellation - as in even order distortion reduction in well balanced diff pair, or push-pull output

apply negative feedback, locally as in degeneration or more globally - and in most situations global feedback can be shown to be more effective when not at least equal to local feedback in distortion reduction

selecting "more linear" amplifying devices is often suggested but somewhat dubious - we basically have the choice of "square law" or exponential gm - and hugely more gain in the exponential gm devices give more linear results with local feedback that reduce them to the same gain as what are often touted as "more linear" devices

IM is created by the same mechanism as harmonic distortion: non-linearity. Increase linearity and you reduce IM. It is as simple, or complicated, as that. IM can be easier to hear and sometimes easier to measure, but it comes from the same mathematics of multiplying trigonometric functions.

You get rid of IMD the same way you get rid of THD: improve linearity. Once you have a linear design, the only other way to reduce IMD is by not dirtying up the signal chain by using good star grounding techniques to keep ripple harmonics out of the signal chain, by isolating heater wiring by twisting the leads and keeping them well away from low level signal lines, by shielding low level lines, and the gNFB feedback connections to the summing node (I like RF coax for this, with the shield braid grounded at the summing node, but not at the other end, so that it acts as a Faraday cage to minimize elctrostatic coupling. You could also include a Zobel at the output to reduce the RF that rides in on the speaker leads. If you use DC heater power, make certain you filter it very well.

The less unnecessary frequencies you introduce, the fewer frequencies there are to mix with desired frequencies to make IMD distortion products.

For reduced bandwidth to work you would have to filter both input to and output from the non-linear section. Even then you will still get odd-order products in band.

Quote:

Originally Posted by Miles Prower

. . . the gNFB feedback connections to the summing node (I like RF coax for this, with the shield braid grounded at the summing node, . . .

You just need to be careful that the cable capacitance in parallel with the summing node does not disturb the frequency response or degrade HF stability. Given that the feedback pickup point is usually low impedance a simple homemade twisted pair should be good enough. To eliminate induction, the ground wire of the pair can double as the ground reference for the output - this means that the output signal seen by the load and the output signal seen by the feedback loop are the same and referenced to the same ground as the input signal.

What is the reason for THD and IMD are not strictly proportional? There are designs where there is ultra low THD but significant IMD (typically solid state with huge GFB) or not-so-good THD and quite low IMD (mostly tube designs).

where's your evidence for that claim? - are you talking a "straw man" example of poorly applied negative feedback - it really can't help an cirucit with deadzone or hysterisis - the circuit must be able to drive the load without "gaps" before feedback

if a circuit has smooth nonlinearities, sufficient slew rate, dynamic capabilities then high (global) negative feedback reduces all orders of distortion

THD and IMD may exist at different frequencies. By definition, the harmonics in THD are higher in frequency than the fundamental. IMD may be higher or lower in frequency. This can create a difference if the circuit has different gain or different feedback at different frequencies.

By IMD I mean intermodulation between signal components. You can also get IM between signal and other things such as hum, RF, PSU noise etc. If this gets lumped in with normal IMD then the result is confusion, as the cause and hence solution is different. To get rid of normal IMD you improve linearity. To get rid of PSU IM you improve PSRR.